From this image, if the Voyager is even
with the nearest face, and given that the ship is 132 metres across, that makes the cube's face about 700 metres square. Just 60 metres more than the Enterprise D's total length.
If the Voyager is pass
the cube, then the cube is smaller than 700 metres. The further away
the Voyager is in this image, the smaller the cube.
The baby "tactical
" cube is much smaller than the large "strategic
" cube that the Enterprise encountered. Each face of that larger cube was multiple times the length of the Enterprise.
These articles make it clear that the tactical cube was an incredibly powerful ship, which had greater firepower than a standard borg cube
And from the star trek online wiki